1. Field of the Invention
This invention relates generally to chemical blowing agent compositions, and more particularly, to activator concentrates useful in combination with a primary chemical blowing agent in foaming polymer resins.
2. Description of Related Technology
Blowing agents are commonly used either alone or in combination to produce a cellular structure in a plastic mass. Generally, blowing agents may be classified either as "physical" or "chemical" blowing agents. Physical blowing agents are generally materials which are normally gaseous at the temperature at which the plastic mass is to be foamed. Chemical blowing agents are generally compositions which decompose or react under the influence of heat to form a gas. Chemical blowing agents range from simple salts such as ammonium or sodium bicarbonate to complex nitrogen-releasing compounds. Chemical blowing agent "concentrates" are compositions or components of compositions which are encapsulated in a carrier resin in a high proportion relative to the carrier resin.
Desirable properties of a chemical blowing agent include:
(a) a narrow gas-release temperature range,
(b) a controllable but rapid gas release rate,
(c) non-corrosive reaction products,
(d) stability in storage, and
(e) reaction products and residue which are compatible with the material to be foamed and have little or no detrimental effect on its properties.
Chemical blowing agents such as described by LaPierre, et al. in U.S. Pat. Nos. 4,769,397 (Sept. 6, 1988) and 4,806,294 (Feb. 21, 1989) include a primary chemical blowing agent, such as sodium borohydride, and an activation system (referred to herein as an "activator"). The activator generates carbon dioxide and water, the latter of which serves as a proton donor which reacts with sodium borohydride to form hydrogen gas which is used for foaming with the carbon dioxide generated by the activator. The primary chemical blowing agent and the activator are each separately encapsulated in a respective polymeric carrier resin to form concentrates, which are then formed into pellets. A mixture of primary chemical blowing agent concentrate pellets and activator concentrate pellets forms a blowing agent concentrate.
Pellets of the activator concentrate, however, sometimes are highly porous, are friable, produce an excessive amount of fines and dust, do not have a uniform dispersion of the activator encapsulated within the carrier resin, and do not have highly uniform pellet size.